It has long been known to separate a variety of gaseous mixtures by cryogenic rectification, for example pretreated air and natural gas. In such processes, the gaseous mixture to be separated is pressurized, purified and then cooled to a temperature suitable for its rectification. The rectification of the gaseous mixture occurs within one or more distillation columns. Each of the columns has mass transfer elements such as trays or packing, for example, structured packing, which bring liquid and vapor phases of the gaseous mixture into contact with one another and effectuate mass transfer between the vapor and liquid phases.
The incoming feed is thereby distilled within the distillation columns or columns to form component streams enriched in the components of the gaseous mixture. The component streams can be taken as liquid and gaseous products and are used in the cooling of the gaseous mixture after having been compressed and purified to a temperature suitable for the separation of the gaseous mixture within the distillation column or columns. The cooling takes place through indirect heat exchange that is conducted in a plant main heat exchanger.
In order to minimize warm end losses in the main heat exchanger and to produce liquid products, refrigeration can be generated by expanding a compressed stream made up of the gaseous mixture and introducing the compressed stream into at least one of the columns in a plant.
It is also known to mechanically pump a liquid product, for instance in air separation, an oxygen-rich liquid column bottoms stream may be vaporized within the same main heat exchanger against a liquefying compressed air stream provided for such purpose.
Given that energy supply costs for electric power consumed in compressing the feed can vary with the time of day, there is a growing incentive to be able to manipulate plant product slates and in particular, liquid production rates. For example, high purity oxygen plants are often designed to produce anywhere of up to about 10 percent of the air as a liquefied product. There exists the need to manipulate the flow of products so that at times less than the maximum capability of the plant is utilized, for example, plant operations in which less than 10 percent of the air is taken as the liquid product. In order to change liquid production rates, it is conventional practice to adjust the turbine flow employed in generating refrigeration. An example of this can be found in U.S. Pat. No. 5,412,953. In this patent, a pumped liquid oxygen plant is described in which the liquid product make is adjusted by adjusting flow to the turboexpander. This adjustment of flow is effectuated by recycling air from the bottom of the higher pressure column to a compressor that is used in compressing the air to the turboexpander. Such operation can result in wide swings in air compression requirements that are required for such purposes as vaporizing pressurized column liquids.
Another possibility in controlling liquid production is to vary the expansion ratio of the turbine expander by increasing or decreasing the pressure of the compressed mixture being introduced into the turboexpander. This also can result in control problems in that as the pressure is increased, the mixture to be expanded may be liquefied at the exhaust of the turbine. In an extreme case where between about 10 and about 15 percent of the compressed process feed is to be liquefied. In such situations, the turbine may suffer from poor efficiency and may incur potential damage. At the other extreme, as pressure is decreased, the temperature of the expanded stream increases when the turbine inlet temperature is relatively fixed by the main heat exchanger design. When such increase is above the saturation temperature of the expanded feed to a column, liquids within the column may vaporize resulting in high local vapor flows, loss of separation performance and potential column flooding.
In the prior art, it is known to control the turboexpander inlet temperature of an air separation plant in order to prevent liquefaction in the turboexpander exhaust. For example in U.S. Pat. No. 3,355,901, a cascade control system is utilized to ensure that the exhaust of a turboexpander used in supplying refrigeration to an air separation plant is at about saturation temperature or slightly superheated. In this patent, warm vapor is divided into two streams. One stream is cooled within a heat exchanger against a cryogenic gas produced in the air separation process and the other stream by-passes the heat exchanger. The streams are then combined and introduced into the inlet of a turboexpander. The turbine exhaust temperature is sensed and a signal referable to such temperature is fed as an input into the cascade control system to control a valve that in turn controls flow of the stream that is cooled within the heat exchanger. However, it is to be noted that such arrangement is to be used in a plant that does not manipulate expansion ratio and as such the variation of turbine exhaust temperature is limited. It could not be used in a plant where expansion pressure and ratio vary substantially.
As will be described, the present invention provides a method and apparatus for separating a gaseous mixture in which refrigeration and therefore liquid production is varied by simultaneous manipulation of turbine expansion ratio and inlet temperatures. Simultaneous manipulation of turboexpander inlet temperature allows for greater variability of liquid production than would otherwise exist by manipulation of turbine expansion ratio alone.